SMTP
Mobile Computing
Prof. Jongwon Yoon
Intelligent Machines Lab.
SMTP overview
•
SMTP clients and servers have two main components
• User Agents (UA): Provides service to the user to make the process
of preparing the message.
• Mail Transfer Agent (MTA): Transfers the mail across the internet
• Analogous to the postal system in many ways
Message Transfer Agent
• The actual mail transfer is done through message transfer
agents (MTAs).
• To send mail, a system must have the client MTA, and to
receive mail, a system must have a server MTA.
• The formal protocol that defines the MTA client and server
in the Internet is called Simple Mail Transfer Protocol
(SMTP).
• Two pairs of MTA client-server programs are used in the
most common situation.
SMTP
•
SMTP also allows the
use of Relays allowing
other MTAs to relay the
mail
•
Mail Gateways are used
to relay mail prepared by
a protocol other than
SMTP and convert it to
SMTP
Format of an e-mail
Command and response
Connection establishment
220 service ready
2
1
HELO: deanza.edu
250 OK
3
Message transfer
Connection termination
1
QUIT
221 service closed
2
Example
Limitations in SMTP
Only uses NVT (Network Virtual Terminal) 7 bit ASCII
format (has limitations)
• No authentication mechanisms
• Messages are sent un-encrypted
• Susceptible to misuse (Spamming, faking sender address)
•
Solution: MIME (SMTP extensions)
•
MIME – Multipurpose Internet Mail Extensions
• MIME transforms non-ASCII data to NVT ASCII data and delivers it
to the client MTA .
• The message at the receiving site is transformed back to the
original data.
MIME Headers
•
Located between the Email Header and Body
• MIME-Version: 1.1
• Content-Type: type/subtype
• Content-Transfer-Encoding: encoding type
• Content-Id: message id
• Content-Description: textual explanation of non-textual contents
MIME Headers
•
Content-Type – Type of data used in the Body
• Text: plain, unformatted text; HTML
• Multipart: Body contains different data types
• Message: Body contains a whole, part, or pointer to a message
• Image: Message contains a static image (JPEG, GIF)
• Video: Message contains an animated image (MPEG)
• Audio: Message contains a basic sound sample (8kHz)
• Application: Message is of data type not previously defined
•
Content-Transfer-Encoding – How to encode the message
• 7 bit – no encoding needed
• 8 bit – Non-ASCII, short lines
• Binary – Non-ASCII, unlimited length lines
• Base64 – 6 bit blocks encoded into 8-bit ASCII
• Quoted-printable – send non-ASCII characters as 3 ASCII characters, =##,
## is the hex representation of the byte
Base64 Encoding
Divides binary data into 24 bit blocks
• Each block is then divided into 6 bit chunks
• Each 6-bit section is interpreted as one character,
25% overhead
•
Quoted-Printable Encoding
Used when the data has a small non-ASCII portion
• Non-ASCII characters are sent as 3 characters
• First is ‘=‘, second and third are the hex representation of
the byte
•
Message Access Agent
• The first and the second stages of mail delivery use SMTP.
• SMTP is not involved in the third stage because SMTP is a
push protocol.
• The third stage needs a pull protocol; the client must pull
messages from the server using a Message Access Agent
(Mail Access Protocol, e.g., POP3, IMAP4).
Post Office Protocol v3
•
•
•
•
•
Simple
Allows the user to obtain a list of their Emails
Users can retrieve their emails
Users can either delete or keep the email on their system
Minimizes server resources
POP3:
Internet
All Messages
Whole
messag
e
POP3
Internet Mail Access Protocol v4
•
•
•
•
•
•
Has more features than POP3
User can check the email header before downloading
Emails can be accessed from any location
Can search the email for a specific string of characters
before downloading
User can download parts of an email
User can create, delete, or rename mailboxes on a server
IMAP:
Dr.Amer
Friends
….
Internet
rs
heade
Web-based E-mail
1
HTTP
transactions
2
3
4
Web-based E-mail
HTTP
transactions
HTTP
transactions
1
2
3